Method of removing heavy hydrocarbons

ABSTRACT

Heavy hydrocarbons contained in FT off gas of a GTL process are removed by bringing the FT off gas into contact with absorption oil, by introducing the FT off gas into a distillation tower, by cooling the FT off gas or by driving the FT off gas into an adsorbent. A burner tip for heating a reformer tube, using FT off gas as fuel, is prevented from being plugged by the deposition of heavy hydrocarbons contained in the FT off gas.

TECHNICAL FIELD

This invention relates to a method of removing heavy hydrocarbonscontained in off gas produced from the Fischer-Tropsch reaction step ofthe gas-to-liquid (GTL) process and utilizing the treated off gas asfuel in the synthesis gas production step of the GTL process.

BACKGROUND ART

Natural gas is regarded as a promising fuel that places less load to theenvironment as compared with petroleum-based fuels because, whencombusted, natural gas gives off neither sulfur oxides nor particulatesubstances that will contaminate the environment and produces lesscarbon dioxide per unit amount of generated heat.

For this reason, natural gas is increasingly attracting attention as analternative fuel that can replace petroleum in the field of energysupply because solutions to the above environmental problems areurgently being looked for and diverse resources are required all overthe world.

The GTL technology is known to provide methods of producing liquidsynthetic hydrocarbons such as naphtha, kerosene, gas oil and so on byway of chemical reactions using natural gas as a raw material. Processesbased on the GTL technology generally include a step of producingsynthesis gas (i.e. a gaseous mixture of carbon monoxide and hydrogen)by way of a reforming reaction (synthesis gas production step), a stepof producing synthesis oil containing straight chain hydrocarbons asmain components from the synthesis gas by way of a Fischer-Tropsch (FT)synthesis reaction (FT synthesis step) and a step of turning thesynthesis oil into product oil by way of a hydrotreating,Hydroisomerization and hydrocracking reaction (upgrading step).

Synthesis gas is produced by way of a reforming reaction of natural gas.Known techniques for producing synthesis gas, utilizing a reformingreaction, include the steam reforming method, the CO₂ reforming method,the autothermal reforming method (ATR), the catalytic partial oxidationmethod (CPDX) and the direct partial oxidation method (PDX).

From the viewpoint of underlying principles, reforming reactions areroughly classified into: steam reforming reaction of producing synthesisgas by adding steam to natural gas, following the reaction equation (1)shown below; and carbon dioxide reforming reaction of producingsynthesis gas by adding carbon dioxide to natural gas or by using carbondioxide contained in natural gas, following the reaction equation (2)shown below. Note that reforming reactions of methane contained innatural gas are represented as examples in the equations shown below.

CH₄+H₂O→CO+3H₂

ΔH₂₉₈=+206 kJ/mol  equation (1)

CH₄+CO₂→2CO+2H₂

ΔH₂₉₈=+248 kJ/mol  equation (2)

As seen from the above equation (1) and (2), both of the above listedreforming reactions are endothermic reactions, meaning that the heatnecessary for carrying out the reactions needs to be externallysupplied. In this regard, in the case of ATR, CPDX and PDX, thereforming reaction is driven to proceed by completely oxidizing a partof the raw material natural gas by means of a burner and/or a catalystand using the heat that is generated when carbon dioxide and water areproduced from hydrocarbons such as methane. Therefore, the overallreaction system is an exothermic reaction and hence no heat needs to beexternally supplied.

In the case of the steam reforming method and the CO₂ reforming method,on the other hand, a reformer tube that is filled with a catalyst isarranged in a furnace and the heat necessary for the reforming reactionis externally supplied by using a heating means such as a burner.

The steam reforming method and the CO₂ reforming method require a largeamount of heat for the reforming reactions particularly when synthesisgas needs to be produced on a large scale. Hence, since a largereforming equipment needs to be installed, a scale merit is hardlyexploited. For this reason, ATR and PDX are believed to be suitable forlarge scale production.

However, both ATR and PDX involve a step for adding oxygen to naturalgas, which requires an oxygen plant that is highly costly. Additionally,since heat is produced to a large extent when oxygen is added to naturalgas, a risk of explosion exists. This means that designing and runningan oxygen plant are subjected to various restrictions.

On the other hand, both the steam reforming method and the CO₂ reformingmethod provide an advantage that, since oxygen is not introduced intonatural gas and their reforming reactions themselves are endothermic,synthesis gas can be produced in safe. Additionally, when the steamreforming method and the CO₂ reforming method are concurrently employed,the hydrogen/carbon monoxide ratio in the produced synthesis gas can bebrought close to 2.0, which is advantageous for the subsequent FTsynthesis reaction.

As for the disadvantage of the steam reforming method and the CO₂reforming method of externally supplying heat, the efficiency of energyuse of the overall GTL process can be improved by reutilizing the FT offgas produced from the subsequent FT synthesis step as fuel for heatingthe reformer tube. FT off gas refers to gas containing the synthesis gasleft unreacted and other gases such as methane that are secondarilyproduced in the FT synthesis step.

In the FT synthesis step, a unit (—CH₂—) of a hydrocarbon chain isproduced from hydrogen and carbon monoxide and such units aresynthetically combined to grow hydrocarbon chain. This reaction is anexothermic reaction as a whole that is expressed by the reactionequation (3) shown below.

(2n+1)H₂ +nCO→C_(n)H_(2n+2) +nH₂O

ΔH₂₉₈=−167 kJ/mol-CO  equation (3)

The number of carbon atoms of the hydrocarbons produced from the FTsynthesis reaction is not fixed and hydrocarbons showing various degreesof polymerization (n numbers) are produced. The ratio of the number ofthe units (—CH₂—) that are actually used for the growth of hydrocarbonchains to the total number of the produced units is referred to as chaingrowth probability α and the extent to which hydrocarbon chains showinga certain degree of polymerization are produced is determined by thechain growth probability α (Anderson-Schulz-Flory distribution). In theFT synthesis step, the reaction is conducted with α>0.85 that boosts theproduction of the kerosene and gas oil fraction, which is the maintarget of the GTL process, or α>0.90 that boosts the production of theheavier wax fraction, from which kerosene and gas oil can be obtained byhydrocracking. However, lighter hydrocarbons that cannot besatisfactorily grown are also produced to a small extent even with an αvalue in the above cited region. Furthermore, not all the synthesis gasfed to the FT synthesis step is consumed and the hydrogen and the carbonmonoxide that are supplied are partly left unreacted.

Hydrocarbons having a large number of carbon atoms and showing a highboiling point that are the target of the FT synthesis reaction are takenout as liquid components, while H₂O and lighter hydrocarbons such asmethane and ethane that are byproducts of the reaction as well asunreacted hydrogen and carbon monoxide are taken out as mixed gaseouscomponents. Of these, hydrogen and carbon monoxide can be reutilized inthe FT synthesis reaction. Therefore, H₂O and other unnecessarysubstances are removed from the mixed gaseous components by cooling andcondensing the gaseous components by means of cooling water and themixture gas containing hydrogen, carbon monoxide, methane, ethane andother low boiling point hydrocarbons that are not condensed are recycledto the FT synthesis reaction. However, since methane and some otherhydrocarbons do not react further even if led back to the FT synthesisreaction, the ratio of methane contained in the gaseous componentsgradually rises. To prevent the methane and some other hydrocarbons frombeing accumulated in the FT synthesis reaction system, part of themixture gas is drawn out and removed as off gas (FT off gas). The offgas produced in this way contains combustible components such as methaneto a large extent and hence it can be used as fuel for heating thereformer tube.

SUMMARY OF INVENTION Technical Problem

However, FT off gas contains a small extent of heavy hydrocarbons thatare the target product of the FT synthesis reaction. Particularly, heavyhydrocarbons having five or more carbon atoms per molecule can bethermally decomposed or polymerized and deposited as liquid or solidsubstances when heated. Therefore, when off gas is used as fuel forheating the reformer tube without being preprocessed, the depositedheavy hydrocarbons can plug a tip of the burner for heating the reformertube to prevent the burner from stably operating for heating andconsequently reduce the efficiency of the GTL process.

Solution to Problem

As a result of intensive research efforts, the inventors of the presentinvention completed this invention as means for solving theabove-identified problem. According to the present invention, there isprovided a method of producing various hydrocarbon oils from natural gasincluding:

a synthesis gas production step of producing synthesis gas containinghydrogen and carbon monoxide as main components by causing natural gascontaining methane as main component to react with steam and/or carbondioxide in a heated reformer tube filled with a reforming catalyst;

a Fischer-Tropsch synthesis step of producing Fischer-Tropsch oil bysubjecting the synthesis gas produced in the synthesis gas productionstep to a Fischer-Tropsch reaction and subsequently separating FT offgas containing gaseous products and unreacted synthesis gas; and

an upgrading step of producing various hydrocarbon oils by subjectingthe Fischer-Tropsch oil to a hydrotreating, hydroisomerization andhydrocracking process, wherein

the FT off gas is recycled as a fuel for heating the reformer tube afterheavy hydrocarbons contained in the FT off gas is removed.

Advantageous Effects of Invention

According to the present invention, heavy hydrocarbons having five ormore carbon atoms per molecule that are contained in the FT off gas areremoved before the FT off gas is supplied to the burner. Thus, theburner tip is prevented from being plugged as a result of thermaldecomposition or polymerization of such heavy hydrocarbons, and thereformer tube is allowed to stably operate for a long period.Additionally, since FT off gas can be utilized effectively as fuel, theefficiency of energy use of the overall GTL process can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of the method of removing heavyhydrocarbons in Example 1.

FIG. 2 is a schematic illustration of the method of removing heavyhydrocarbons in Example 2.

FIG. 3 is a schematic illustration of the method of removing heavyhydrocarbons in Example 3.

FIG. 4 is a schematic illustration of the method of removing heavyhydrocarbons in Example 4.

FIG. 5 is a schematic illustration of the method of removing heavyhydrocarbons in Example 5.

DESCRIPTION OF EMBODIMENTS

Embodiments of a method of recycling FT off gas as fuel for a synthesisgas production step according to the present invention will be describedbelow.

The synthesis gas produced by means of a reforming reaction is driven toflow into a bubble tower reactor installed on the way of the FTsynthesis step from the bottom thereof. The bubble tower reactor isfilled with slurry consisting of liquid hydrocarbons produced as aresult of an FT synthesis reaction and catalyst particles. When thesynthesis gas rises through the slurry contained in the tower,hydrocarbons are produced as a result of an FT synthesis reactionbetween carbon monoxide and hydrogen gas.

Of the synthesized hydrocarbons, liquid ones are led into a separator asslurry along with catalyst particles. The solid components such ascatalyst particles and the liquid components including liquidhydrocarbons are separated from each other in the separator. Theseparated solid components are then returned to the bubble towerreactor. The liquid components, on the other hand, are supplied to adistillation tower and heated for fractional distillation of producing anaphtha fraction (boiling point: about 150° C. or lower), a kerosene/gasoil fraction (boiling point: about 150° C. to 350° C.) and a waxfraction (boiling point: about 350° C. or higher). Subsequently, eachfraction is fed to the upgrading step.

On the other hand, the gaseous components including unreacted synthesisgas and synthesized gaseous hydrocarbons are discharged from the top ofthe bubble tower reactor and supplied to a hydrocarbon collector. Anytype of hydrocarbon collector can be used. A hydrocarbon collector to beused herein typically cools the gaseous components by means of a heatexchanger using cooling water and separates the liquid componentsincluding condensed water and liquid hydrocarbons from the gaseouscomponents that are left uncondensed. Among the separated liquidcomponents, water is removed and liquid hydrocarbons are led into thedistillation tower. The gaseous components left in the hydrocarboncollector after the separation mainly include unreacted synthesis gasand light hydrocarbons such as methane and ethane, but they also includeheavy hydrocarbons, which are also left there by a small amount. Thegaseous components are reintroduced into a bottom section of the bubbletower reactor and reutilized for the FT synthesis reaction. At thistime, a part of the reintroduced gaseous components is drawn out anddischarged as off gas (FT off gas) in order to prevent accumulation ofmethane, ethane and the like which does not react further in the bubbletower reactor, and the FT off gas is utilized as fuel gas for heatingthe reformer tube for the reforming reaction in the synthesis gasproduction step.

However, thus discharged FT off gas that is to be recycled and utilizedas fuel gas for the reforming reaction also contains heavy hydrocarbonsby a small amount. When off gas containing such heavy hydrocarbons isutilized as fuel gas, the contained heavy hydrocarbons are deposited byway of thermal decomposition and polymerization and plug the burner tipto become an obstacle for the operation of stably heating the reformertube for a long period. According to the present invention, this problemis dissolved by removing the heavy hydrocarbons contained in FT off gasbefore the FT off gas is utilized as fuel for heating a reformer tube.

Now, the present invention will be described in more detail by way ofpreferred embodiments of the method of removing heavy hydrocarbonsaccording to the present invention. Note, however, that the presentinvention is by no means limited to the following embodiments.

The first embodiment of the present invention is a method of bringing FToff gas into contact with absorption oil directly in order to absorb theheavy hydrocarbons contained in FT off gas by the absorption oil.

While any oil can be used as the absorption oil so long as it can absorbhydrocarbons having five or more carbon atoms per molecule contained inFT off gas, relatively light kerosene or gas oil may preferably be used.The kerosene fraction or the gas oil fraction that is a final product ofa GTL process, for example, can be used. Alternatively, if appropriateintermediate oil is available in a GTL process, such oil may be used.The kerosene or gas oil fraction obtained as a result of fractionaldistillation by a distillation tower or a product obtained byhydrotreating such a fraction may be used as the intermediate oil.

Similarly, the conditions of absorption are not subjected to anyparticular limitations and it can be appropriately selected depending onthe absorption oil to be used and other considerations. However, thetemperature is preferably between 10° C. and 50° C. and close to theroom temperature and the pressure is preferably between 2.4 MPaG and 3.2MPaG from the viewpoint of preventing absorption oil from evaporating.

The second embodiment of the present invention is a method ofintroducing FT off gas into a distillation tower and distilling andseparating the heavy hydrocarbons contained in the FT off gas.

Any distillation tower can be used so long as it can separatehydrocarbons having five or more carbon molecule per molecule from theFT off gas introduced in it by distillation. A distillation tower may beadditionally installed for a GTL process that embodies the presentinvention. However, if a distillation tower that can separatehydrocarbons having five or more carbons per molecule from the FT offgas introduced therein exists in a GTL process, the FT off gas may beintroduced in it. For example, a distillation tower is normally operatedto separate the product (hydrogenated naphtha), obtained byhydroprocessing the naphtha fraction drawn from the top of adistillation tower, into liquid naphtha and gas containing lighthydrocarbons as main components. Then, FT off gas may be introduced intosuch a distillation tower and the gas obtained as a result of suchseparation may be used as fuel for heating a reformer tube.

While the conditions of distillation are not subjected to any particularlimitations, preferably the operation of distillation is conducted witha temperature between −50° C. and 40° C. and a pressure level between2.4 MPaG and 3.2 MPaG at the tower top.

The third embodiment of the present invention is a method of cooling FToff gas and condensing and separating the heavy hydrocarbons containedin it.

While the cooling conditions are not subjected to any particularlimitations so long as the hydrocarbons having five or more carbon atomsper molecule contained in FT off gas are separated under the selectedconditions, preferably the temperature is between 5° C. and 20° C. andthe pressure is between 2.4 MPaG and 3.2 MPaG for example. Similarly,any cooling method can be used. For example, cooling water or a heatexchanger using water discharged from the GTL process may be used.

The fourth embodiment of the present invention is a method of removingthe heavy hydrocarbons contained in FT off gas by adsorption.

While any adsorbent can be used so long as it can separate thehydrocarbons having five or more carbon atoms per molecule contained inFT off gas, active carbon can be employed to adsorb the hydrocarbons forinstance. When active carbon is employed, the operating condition of anadsorption is preferably such that the temperature is between 20° C. and40° C. and the pressure level is between 0 MPaG and 3.2 MPaG.Additionally, the active carbon is preferably regenerated by way of asteaming treatment that is conducted at a temperature between 100° C.and 150° C. and a pressure level between 0 MPaG and 0.35 MPaG. With thismethod, two adsorption towers may preferably be installed in a GTLprocess and operated alternately for adsorption and regeneration. Then,the adsorption towers can be operated continuously.

Any of the above-described first through fourth embodiments may notnecessarily be adopted alone. In other words, the advantages of thepresent invention can be boosted when, for example, FT off gas isbrought into contact with absorption oil (the first embodiment) or anadsorbent is applied thereto (the fourth embodiment) while the FT offgas is being cooled (the second embodiment).

Now, the present invention will be described by way of examples for thepurpose of better understanding of the present invention. However, thepresent invention is by no means limited by the examples.

Example 1

Gas produced from an FT synthesis step in a plant using GTL technology,that includes unreacted synthesis gas and gaseous hydrocarbons, wascooled and the gas phase and the liquid phase were separated from eachother to obtain FT off gas having a composition shown in Table 1. Notethat, in Table 1, Cn denotes hydrocarbons having n carbon atoms permolecule and C5+ denotes hydrocarbons having five or more carbon atomsper molecule. The temperature and the pressure of the obtained FT offgas was respectively 45° C. and 2.75 MPaG and the flow rate of the FToff gas was 1,000 kmol/h (=15,284 kg/h). The content of the heavyhydrocarbons having five or more carbon atoms per molecule was 4.32 wt%.

TABLE 1 Components Composition ratio of FT off gas (mol %) H₂ 27.29 N₂0.16 CO 15.56 CO₂ 0.58 H₂O 0.45 C1 53.40 C2 0.39 C3 0.81 C4 0.48 C5+0.79 Total 100.00

The obtained FT off gas was cooled to 10° C. under a pressure level of2.7 MPaG and the condensed liquid components were separated from theuncondensed fuel gas. Then, the heavy hydrocarbons contained in theliquid components were collected (FIG. 1). Subsequently, the content ofthe heavy hydrocarbons having five or more than five carbon atoms permolecule contained in the fuel gas were measured.

Example 2

The FT off gas obtained in Example 1 was introduced into a distillationtower and the fuel gas drawn from the tower top and the liquidcomponents drawn from the tower bottom were separated from each other(FIG. 2). The pressure of the FT off gas at the tower top was 2.65 MPaGand the temperature at the tower bottom was 198° C. while thetemperature at the tower top was −37° C. Thereafter, the content of theheavy hydrocarbons having five or more carbon atoms per moleculecontained in the fuel gas that was obtained at the tower top wasmeasured.

Example 3

The FT off gas obtained in Example 1 was brought into contact withabsorption oil that was equivalent to gas oil in a single stage underconditions of a pressure level of 2.7 MPaG and a temperature of 48° C.of the FT off gas and the heavy hydrocarbons absorbed in the absorptionoil were collected (FIG. 3). Table 2 shows the distillation property ofthe employed absorption oil. The flow rate of the absorption oil was10,000 kg/h. Thereafter, the content of the hydrocarbons having five ormore carbon atoms per molecule that were contained in the fuel gas andnot absorbed by the absorption oil was measured.

TABLE 2 Distillation (%) (° C.) 5 250 10 264 20 271 50 296 80 335 90 36195 382

Example 4

The temperature at which the FT off gas was brought into contact withabsorption oil in Example 3 was lowered to 10° C. and the heavyhydrocarbons were collected (FIG. 4). The flow rate of the absorptionoil was 10,000 kg/h. Thereafter, the content of the heavy hydrocarbonshaving five or more carbon atoms per molecule that were contained in thefuel gas and not absorbed by the absorption oil was measured.

Example 5

The FT off gas same as that of Example 3 was brought into contact withabsorption oil by means of an 8-stage absorption tower to collect theheavy hydrocarbons (FIG. 5). The pressure of the FT off gas at the towertop was 2.65 MPaG and the temperature at the tower bottom was 49° C.while the temperature at the tower top was 50° C. The flow rate of theabsorption oil was 10,000 kg/h. Thereafter, the content of the heavyhydrocarbons having five or more carbon atoms per molecule that werecontained in the fuel gas and not absorbed by the absorption oil wasmeasured.

Examples 6 Through 8

In Examples 6 through 8, experiments respectively same as those ofExamples 3 through 5 were conducted except that the flow rate ofabsorption oil was doubled to 20,000 kg/h.

Table 3 shows the content of the heavy hydrocarbons having five or morecarbon atoms per molecule contained in the fuel gas that was obtainedafter removing heavy hydrocarbons, the flow rate of the collectionliquid or the absorption oil and the ratio by which the heavyhydrocarbons contained in the FT off gas were distributed to thecollection oil or the absorption oil of each of Examples 1 through 8. Ineach of the examples, the effect of removing heavy hydrocarbons fromfuel gas was proved.

TABLE 3 flow rate of distribution of heavy fuel gas collectionhydrocarbons content of liquid/ to to collection flow heavy absorptionfuel liquid/absorption rate hydrocarbon oil gas oil (kg/h) (wt %) (kg/h)(wt %) (wt %) Ex. 1 14940 2.34 344 52.9 47.1 Ex. 2 14627 0.06 657 1.498.6 Ex. 3 14716 1.52 10568 33.9 66.1 Ex. 4 14411 0.53 10873 11.5 88.5Ex. 5 14550 0.53 10734 11.7 88.3 Ex. 6 14544 1.02 20740 22.6 77.4 Ex. 714237 0.32 21047 6.8 93.2 Ex. 8 14362 0.09 20922 2.0 98.0This application claims the benefit of Japanese Patent Application No.2011-078804, filed Mar. 31, 2011, which is hereby incorporated byreference herein in its entirety.

1. A method of producing various hydrocarbon oils from natural gascomprising: a synthesis gas production step of producing synthesis gascontaining hydrogen and carbon monoxide as main components by causingnatural gas containing methane as main component to react with steamand/or carbon dioxide in a heated reformer tube filled with a reformingcatalyst; a Fischer-Tropsch synthesis step of producing Fischer-Tropschoil by subjecting the synthesis gas produced in the synthesis gasproduction step to a Fischer-Tropsch synthesis reaction and subsequentlyseparating FT off gas containing gaseous products and unreactedsynthesis gas; and an upgrading step of producing various hydrocarbonoils by subjecting the Fischer-Tropsch oil to a hydrotreating,hydroisomerization and hydrocracking process, wherein the FT off gas isrecycled as a fuel for heating the reformer tube after heavyhydrocarbons contained in the FT off gas is removed.
 2. The methodaccording to claim 1, wherein the heavy hydrocarbons are hydrocarbonshaving five or more carbon atoms per molecule.
 3. The method accordingto claim 1, wherein the heavy hydrocarbons are removed by bringing theFT off gas into contact with absorption oil directly.
 4. The methodaccording to claim 3, wherein the absorption oil is one of kerosene andgas oil.
 5. The method according to claim 1, wherein the heavyhydrocarbons are removed by distilling the FT off gas.
 6. The methodaccording to claim 1, wherein the heavy hydrocarbons are removed bycooling the FT off gas.
 7. The method according to claim 1, wherein theheavy hydrocarbons are removed from the FT off gas by adsorption.
 8. Themethod according to claim 7, wherein an adsorbent used for theadsorption is active carbon.